DE 195 39 616 A1 relates to a cylinder-piston rod unit that is filled with fluid, and in particular a gas spring, the piston of which is provided with spring tongues that have rounded ends that engage resiliently the inner wall of a cylinder, while the rest of the piston is free from engagement with the cylinder. The spring tongues are shaped so that before assembly of the piston into the cylinder their radially outermost tips are spaced apart from the piston axis by a distance that is greater than the radius of the cylinder and after assembly bear against the inner wall of the housing with a predetermined prestress. For damping the movement of the piston near the end of its extension stroke (movement out of the cylinder), the cylinder has a groove that narrows near the rod end of the cylinder.
It is possible for the spring tongues, in the assembled piston/cylinder, to become subject to friction forces higher than those created by the prestress alone as a result of the effect of friction, together with rotation of the piston in a direction in which the tips of the tongues "lead" with respect to the roots of the tongues. Similarly, the guiding effect of the tongues may be reduced in the event of rotation of the piston in a direction in which the tips of the tongues trail the roots as a result of friction in the circumferential direction producing forces on the tongues that counteract the radially outward prestress. The above effects may be even more pronounced when a spring tongue gets stuck in the longitudinal damping groove in the cylinder. Furthermore, a spring tongue may become caught in the longitudinal groove in the cylinder and reduce the cross section of the groove, so that the intended function of the groove is no longer performed.
There is a critical moment in assembly of the piston/cylinder when the piston rod, with the piston, is introduced into the cylinder. Care must be taken to see that the piston ring is not pinched along the edge of the open cylinder. That possibility exists to a considerable degree in the piston ring of DE 195 39 616 A1.
U.S. Pat. No. 2,992,052 discloses a self-sealing piston carried by a piston rod and received for axial movement in a cylinder. The inner surface of the cylinder is engaged by part of the outer surface of the piston, namely, a radially resilient peripheral wall portion, which extends continuously around the circumference of the piston and provides tolerance compensation with respect to the inside diameter of the cylinder. The peripheral wall portion forms with an external surface of the body of the piston an annular recess or groove, which provides space for deformation of the peripheral elastic wall portion.
One disadvantage of the piston of U.S. Pat. No. 2,992,052 is that even small variations in the thickness of the elastic wall portion can lead to significant differences in the elasticity in the wall, owing to which the amount of friction can be restricted only with great difficulty. The elastic wall portion is structurally a ring, and deformation requires both compressive stresses in the circumferential direction as well as bending in the radial direction.
In addition, a tapered transition is present at the transition from the elastic wall to the piston body. This tapered transition represents an active hydraulic or pneumatic surface, the effect of which must be taken into account. Depending upon the direction from which an operating pressure acts, a compressive force component builds up which allows the elastic wall to lift away from or press more strongly on the inside diameter of the cylinder.